This invention relates generally to sonic tools and more particularly relates to improvements in the system for exciting such tools.
A sonic tool is a device which is excited into mechanical vibration at sonic or ultrasonic frequencies in order to perform useful work. Such tools are used for heating, drilling, cutting, sawing or deforming a work piece. Sometimes the tool is provided with a variety of interchangeably attachable tool members.
Sonic tools generally consist of an electronic drive circuit which generates electronic oscillations for exciting a sonic transducer to which the tool member is mounted.
A sonic transducer typically comprises an elongated metallic body with an interposed magneto-strictive transducing element or an interposed piezoelectric transducing element which is excited by electronic drive circuitry. The transducing elements advantageously consist of a stack of piezoelectric wafers which are mounted coaxially with the driven metallic portion of the sonic transducer and biased under longitudinal compression. Such a stack arrangement is illustrated in U.S. Pat. No. 3,889,166.
Electrical excitation of the drive transducing element generates mechanical compression waves which, at the natural frequency of vibration or resonance of the entire body, produce a standing wave pattern along the longitudinal axis. This standing wave pattern defines nodes and antinodes along the mechanical body. The nodes are positions of maximum strain or deformation and minimum axial, linear displacement of the vibrating body. The antinodes are positions of minimum strain and maximum reciprocating displacement. Sonic tools are therefore ordinarily designed so that a piezoelectric transducing element is positioned at a node and the working surface of the tool member is positioned at an antinode.
Various problems arise with the design of such tools. One of the chief problems arises because the frequency, which is necessary to excite the tool in a manner which gives a maximum displacement at the working surface of the tool member, shifts as the tool is loaded by contact with the workpiece and may also shift as a result of the use of different interchangeable tool members or tips. It has furthermore been observed that not only is the frequency of resonance shifted by loading but further that most sonic tools have, in addition to the desired frequency of resonance, several spurious or parasitic resonant frequencies. These are not useful because they generate greatly reduced displacement of the working surface of the tool member.
Therefore, many different electronic driving circuitry systems and schemes have been proposed for controlling and modifying the excitation frequency of the sonic transducer of the sonic tool during use. All these problems are further complicated by the high quality factor or Q exhibited by sonic transducers.
One prior art system which has been proposed for controlling the excitation frequency of the circuit driving a sonic transducer is to design the entire sonic tool as an oscillator. A sonic transducer is energized with an amplifier which includes the sonic transducer as the frequency determining element in a feedback loop to provide the closed loop necessary for a conventional oscillator. Such a system is designed on the theory that is is basically a conventional oscillator but has the sonic transducer in the feedback loop instead of the usual tuned circuit constructed of electronic elements. Such systems are shown in U.S. Pat. Nos. 3,474,267 and 3,813,616.
However, such feedback systems require the inclusion of an electronic bandpass filter in the closed loop so that the tool can not be excited at the spurious, unwanted resonant frequencies. Such filters must exhibit a narrow bandpass but in addition to their sharply peaked amplitude characteristic they unfortunately also exhibit an undesirable phase shift characteristic. While exhibiting zero phase shift at its center frequency, the filter will introduce a frequency dependent phase shift into the circuit loop at frequencies removed from the center frequency. This phase shift will be between +90.degree. and -90.degree. .
According to conventional oscillator theory, a total 360.degree. phase shift is required around the closed loop for resonance. The tool will therefore be driven at the frequency which gives a 360.degree. total phase shift around the loop. This is the resonant frequency of the entire closed loop rather than the natural frequency of vibration or resonant frequency of the sonic transducer itself. Consequently, the phase shift introduced in the electronic circuit by the filter causes excitation at a frequency removed from the resonant frequency of the mechanical system. The ultimate result is a reducted or zero amplitude or displacement at the working region of the sonic tool.
Other systems attempts to control the excitation frequency by monitoring and comparing the phase relationship between the voltage and current at the drive transducing element by which the mechanical vibrations are generated. Such systems are illustrated by U.S. Pat. Nos. 2,917,691; 3,778,648; and 3,819,961.
These systems operate under the assumption that, at the mechanical resonant frequency of the body, the voltage and current exhibited at the drive transducing element will be in phase and consequently exhibit electrical resonance. Simply stted, these systems assume that mechanical resonance and electrical resonance observed at the electrodes of the drive transducing elements are coincident.
It has been found that, while this assumption is correct when a sonic transducer is not loaded and consequently is doing no work, the assumption in incorrect as the tool is loaded down. In fact it has been found that the frequency of mechanical resonance and the frequency of electrical resonance progressively diverge as the tool is progressively loaded. Consequently, the system becomes progressively less effective as the tool is loaded because it excites the sonic transducer at its exhibited electrical resonance rather than at its mechanical resonance. We have found that, as such tools are loaded, the displacement of the working surface of the tool is reduced and ultimately ceases.
The prior art has also taught a variety of other circuit systems for exiciting a sonic transducer including systems for hunting the resonant frequency. There is however, a need for a system which will permit the sonic transducer to be energized at the resonant frequency of its mechanical body as that resonant frequency is caused to shift under load rather than being energized at some ineffective approximation to the mechanical resonant frequency.